The Neuroscience of Learning & Memory: Part II

Hi, I’m David Handel, MD, retired radiologist and co-founder of iDoRecall.com. This is Part II of our series on the neuroscience of learning and memory. In Part I, we laid the foundation of basic neuroscience that you’ll need in order to appreciate the more complex and very exciting recent advances we’ve made in understanding how our brain learns, creates new memories and is able to recall memories. If you don’t have a basic understanding of Neuroscience 101, I suggest that you read Part I before proceeding.

Let’s begin by establishing the central role of learning and memory in the evolution of the species and individual survival. This was most eloquently stated by Alcino Silva, PhD., Professor of Neurobiology and principal investigator of the Silvalab at UCLA.

Memory and learning are at the heart of evolution. Evolution is about shaping creatures that can face a challenge and change their behavior so that they’re better equipped to face that challenge again… So it’s not surprising that 70% of all genes… expressed in the brain… a majority of those are involved in memory. This is the thing that evolution has shaped us to do.Alcino Silva, PhD.

Clearly, without our ability to learn and remember, we’d fail to survive as individuals and as a species.

Let’s now direct our attention now to the cognitive psychology of learning and memory.

What are the fundamental types of memory? In Part I, we talked about procedural memory, also known as nondeclarative or implicit memory. As you will recall, this is the memory of physical skills and habits which we develop by repeated and often deliberate practice. We’re talking about skills such as walking, phonation, riding a bike and swinging a golf club. As we consciously learn these skills, they become ingrained in our subconscious and move from long-term storage in the cerebral cortex, where they’d ordinarily be accessible to conscious recall, into the cerebellum where they reside as automatic, subconscious skills. You will recall that once these skills become subconscious, they are very difficult to unlearn as was demonstrated in the hilarious video about how effortful it is to unlearn how to ride a bike.

The other top-level category of memory is declarative or explicit memory. This is the memory that most of us are referring to when we think about memory. It’s the ability to store and retrieve the memory of facts, knowledge and autobiographical experiences. Declarative memory can be subdivided into semantic memory, the accumulation of facts and knowledge we acquire during our lives, and episodic memory, the autobiographical stories and events of our lives that we’re able to consciously recall.

We learned in Part I that conscious declarative memory is created by and stored in various portions of the cerebrum. This type of memory can further be considered from the perspective of whether it’s short-term memory, working memory or long-term memory.

The construct of short-term memory represents the idea of our ability to hold a small and limited amount of new information, such as a telephone number, in conscious memory for a limited period of time. The classic teaching is that we are only able to hold 7 (+/- 2) new things, such as digits, in short-term memory. This school of thought also holds that these memories quickly fade, (in a few seconds, up to half a minute) unless they are converted into long-term memories. If this construct is correct, it’s no wonder we can briefly memorize 7 digit phone numbers and need to rush and dial it before the memory evaporates. Understand that there is much controversy in the research literature about the certainty and nature of short-term memory. The creation of short-term memory undoubtedly begins in the hippocampus. When that structure is destroyed, we are unable to create new anterograde long-term memories. This was the case with the well-studied patients Henry Molaison and former British orchestra conductor Clive Wearing, whose short-term memory only lasted about 7–30 seconds. Neither had any ability for conversion of short-term memory to new a long-term memory after their acquired traumatic event.

Clive Wearing. The man who can’t create new long-term memories.

Working memory is also a construct of cognitive psychology, which also has its share of controversy and opposing opinions. Think of working memory as a system we use to hold and consciously process long-term memory, as well as a buffer where we process short-term memory before it is connected to existing schemas or mental models, and then is committed to long-term memory. Thus working memory is where we create mental models and “wrap our head around” concepts, ideas, and other subject matter. As we mentally play with a concept in our working memory, we utilize many skills such as organization of material, manipulating verbal and visual information, goal completion and other features of our executive function. Much of this occurs in the prefrontal cortex but other areas of the cerebral hemispheres are also involved. This has been confirmed by functional MRI studies. The capacity of our working memory is limited and varies from person to person. We can only actively hold and manipulate a modest number of chunks of information in working memory at one time. This skill requires our attention and focus. Unfortunately, our working memory is a function that becomes more limited as we age. It’s also been shown that the measurement of working memory, at the start of formal education in children, is a more accurate indicator of future academic success than IQ.

Long-term memory is memory which remains stored indefinitely, possibly for as long as the portions of the brain holding the memory remain intact. The procedural memory for skills such as walking, stored in the cerebellum, are a subset of long-term memory. But here we’ll focus on declarative long-term memory, such as memory for facts, knowledge and autobiographical events, which we can recall and play with in our working memory. It is long-term memory which is critical to our evolution, personal survival and ability to learn from our life events. (See Figure 1)

Figure 1. Here the dog likely learned in the past (has a long-term memory): Don’t mess with a skunk, even if he is brazenly eating your lunch!
“Those who cannot remember the past are condemned to repeat it.” –George Santayana

Long-term memory at times requires our focus and attention. At other times, these memories seem to simply drift into consciousness. Memories are usually chained to other memories, contexts and events. Because of these associations, we regularly participate in the universal human experience of recalling one memory which then involuntarily leads to retrieval of another, through sometimes seemingly inapparent associations that connect the two. Later on in our review, we’ll explain how neuroscientists believe this happens at the cellular level.

The conversion of short-term into long-term memory occurs by a process termed memory consolidation. This is a fascinating process whereby memories, which are initially stored in the hippocampus, are transferred to the cerebral cortex for long-term storage. Consolidation can be broken down into two processes — synaptic consolidation (synaptic neuroplasticity) and systems consolidation. Synaptic consolidation is a rapid process occurring in minutes to hours while systems consolidation typically takes weeks to months. Systems consolidation is a process whereby short-term memories in the hippocampus become independent of that structure and are transferred to the cerebral cortex. It has also been shown that systems consolidation can occur extremely quickly, possibly within a few hours rather than weeks, if there exists a framework of related learning (a schema or mental model), with which the new memory can be interleaved and linked.

In recent years, we’ve come to discover that the long-held belief that adult brains can no longer give birth to new neurons is incorrect. In fact, throughout our lives, we develop new neurons in our hippocampus, a process known as neurogenesis. Research has shown that this neurogenesis in the hippocampus is intimately related to the systems consolidation process of long-term memory formation. The amazing part of this story is that as we move our long-term memory storage from the hippocampus to the cerebral cortex, we erase and clear the hippocampus of these memories. It is believed that the process of neurogenesis in the hippocampus causes the forgetting of new memories in that structure as these newborn neurons wire themselves into the neural network of the hippocampus. At the same time, the newborn neurons stimulate the consolidation of these long-term memories in the cortex during sleep and wakeful inactivity though the initiation of sharp-wave ripple complexes (SWRC), which can be seen on electroencephalograms (EEGs). This evidence of interaction between the hippocampus and cerebral cortex that results in consolidation of long-term memory is believed to be the direct result of hippocampal replay of these memories during sleep and quiet awake states, stimulated by the SWRCs.

Reconsolidation of long-term memories is the process of recall of these memories, either spontaneously or by effortful recall, which then results in the strengthening of these memories, or integration of new information into the schemata of these memories. New insight suggests that reconsolidation is really just a part of the never-ending process of consolidation, the natural life-cycle of long-term memory where recall, reminders or encounters with new information causes consolidation and weaving of that new information into the framework and neural circuits of existing mental models.

We’ve talked a lot here about how we create memories and learning on a macro level, but this raises deeper questions of how do these processes manifest on a cellular and molecular level? Come along with us on this journey in Part III of this series where we’ll learn about these amazing and very recent discoveries of the cellular and molecular basis of learning and memory.

In the meantime, please consider signing up for early access to iDoRecall. We’re going to use scientifically-proven principles to help you remember everything you learn, for as long as you want.